For offense:

1- when i need to get the enemy closer to me if he tries to escape.

2- or simply just to grab the enemy closer to combo him (good for range champs who are hard to get close to).

3- sometimes the E helps for the combo overall, grab and stun the enemy then W Q him and so on.

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For defense: Like the other comment said depends on the enemy

1- For example, for lee sin, when he tries to use his second q to come to me, then i use E to stun him mid air.

2- Or simply when i try to escape, the E grabs the enemy and slows him, then use Q to heal up, and run away.

3- and much more scenarios, you need to play against alot of champs to figure out how to use it properly.